GENERAL DISCUSSION 335 



synthesis and the beginning of mitosis. In fact, a fnrther delay in mitosis 

 was not observed. This means that the DNA synthesis and the mitosis 

 were only postponed by 12 hr. Bnt it does not mean that the DNA was 

 not damaged, becanse in a large percentage of the cells embarking on 

 division chromosome breakages were discovered (see the data of 

 Roller). 



bacq: III. We now have to discnss the question of the influence of 

 various factors on the repair of nuclear injuries. 



tummerman: The question is completely incomprehensible. Wherein 

 lies the difference between a natural metabolic process and an artificial 

 process? 



bacq: I will be glad to explain this to you. 



If an increase in the frequency of mutation depends upon the power 

 of the dose, this in my opinion proves that when the radiation is of 

 low intensity repair of the damaged genetic material occurs. 



And once more I have put forward a somewhat provocative notion. 

 Dr. Hollaender does not agree? 



hollaender: I want to say that, in the phenomena discussed by us, 

 repair of the genetic material does not occur. But is there an interference 

 in the chain reaction leading to the appearance of mutations? 



There is one point. Many potential injuries have been shown in 

 experiments with i.r. rays, which by themselves do not have any marked 

 effect, but in combination with X-irradiation augment the effect. 



DUBININ: Some critical remarks on the problem of repair. Experi- 

 mental data on the processes of genetic repair (after fractionated 

 small doses of radiation) has been presented by Nonbak and Auerbach, 

 and also by Luning on Drosophila and by Russel on mice. The new 

 data in this case consist in that the dependence of the frequency of 

 the mutations upon the intensity of the radiation has seemingly been 

 shown in a group of mutations which are recognized to be gene varia- 

 tions, whereas this dependence was known only for chromosome 

 recombinations. 



If we consider the latter, then the restoration of the original structure 

 of the ruptured chromosome can be carried out by two, in principal 

 different, ways. Either it is a repair of the primary damage when the 

 potential rupture is not realized, or it is a repair after the occurrence 

 of chromosome breakage by combination of the fragments which have 

 arisen. 



